Set *sk_resp to NULL when freeing.

Set *sk_resp to NULL when freeing.

Its possible for get_ocsp_resp_from_responder to return OK after having
freed *sk_resp without setting the freed pointer to NULL, leading us to
set a garbage pointer in other code.

Ensure that we set it to NULL after freeing

Addresses https://scan5.scan.coverity.com/#/project-view/60762/10222?selectedIssue=1659225

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28101)